1. Field of the Invention
The invention relates to an electrohydraulic, two-stage, proportional displacement valve having a follower piston system.
2. Description of the Prior Art
It is well known that proportional displacement valves occupy a position midway between ordinary displacement valves and servo valves, with regard to technical value and cost. There are virtually no known proportional displacement valves which employ special servo valve advance control stages, e.g., which may operate with systems comprising a nozzle and deflecting plate. However, advance controls are in common use which operate with two proportional pressure control valves or two proportional pressure limiting valves. The control pressures are varied in proportion to the electrical input signals, and the pressures displace the main control piston with respect to centering springs. There is no positive coupling between the advance control stage and the main control stage. Depending on friction effects and changes in the springs, reproducibility may be low and hysteresis high.
It is also known that in the event of pressure failure in follower piston systems there is a tendency for the system to fall out of adjustment, i.e. to assume an undefined position, unless special hydraulic control systems and/or other centering means for the follower piston are provided. If the drive means of the advance control piston, which drive means may be, e.g., a stepping motor, are also intended to be centered in a prescribed null position, it is desirable for the purposes of facilitating assembly and set-up and lowering manufacturing cost that means be provided for relative adjustment of the null position of the drive and the null position of the control edges, or surfaces, when, as a precondition, the follower piston itself is centered.